1. Technical Field
The present invention relates to a silicon single crystal produced by Czochralski method (hereinafter occasionally referred to as xe2x80x9cCzochralski methodxe2x80x9d, xe2x80x9cCZ methodxe2x80x9d, or a pulling method), which is especially useful for material of solar cell, a method for producing it and a silicon single crystal solar cell produced using it.
2. Background Art
First, characteristics of a solar cell will be explained, in relation to a substrate material constituting the solar cell. The solar cell can be roughly classified on the basis of material of the substrate, into three types of silicon crystal solar cell, amorphous silicon solar cell, and compound semiconductor solar cell. The silicon crystal solar cell can be further classified into single crystal solar cell and polycrystal solar cell. The solar cell having the highest conversion efficiency that is the most important characteristics as a solar cell is the compound semiconductor solar cell among them, of which conversion efficiency is almost 25%. However, it is difficult to produce a compound semiconductor that is material of the compound semiconductor solar cell, and the compound semiconductor solar cell has a problem of cost for production, so that it cannot be generally and widely used. Accordingly, it can be used only for a limited purpose.
In the following description, the word xe2x80x9cConversion efficiencyxe2x80x9d means xe2x80x9cthe rate of energy that can be taken out by being converted to electric energy with a solar cellxe2x80x9d, that is represented as a percentage (%).
The solar cell having the highest conversion efficiency except the compound semiconductor solar cell is silicon single crystal solar cell, of which power generation efficiency is about 20% that is close to conversion efficiency of the compound semiconductor solar cell. The substrate for the silicon crystal solar cell can be prepared relatively easily. Accordingly, it is a major type of solar cell used generally. Furthermore, silicon polycrystal solar cell and amorphous silicon solar cell or the like have also been used practically, since the material of the substrate for them can be produced at low cost, although conversion efficiency of them is about 5 to 15%, which is lower than that of the above-mentioned two types of solar cell.
Secondly, a general method for producing a silicon single crystal solar cell will be explained below. First, in order to make silicon wafer to be a substrate of a solar cell, a columnar silicon single crystal ingot is produced according to Czochralski method or a floating zone melting method (hereinafter occasionally referred to as FZ method, Floating zone method). Then the ingot is sliced to give a thin wafer having a thickness of, for example about 300 xcexcm, and a mechanical damage on the surface of the wafer is removed by etching with chemical to provide a wafer (substrate) that is to be a solar cell. PN junction is formed on one side of the wafer by a diffusion treatment of impurity (dopant), and thereafter electrode is formed on both surface of the wafer, and finally an antireflection coating film is formed on the surface which gets sunbeam in order to reduce loss of optical energy due to reflection of light, and thereby a solar cell is produced.
Nowadays, a demand for a solar cell as one of clean energy is increased to solve the environmental problems. However, its energy cost is higher than common commercial electric power, which is an obstacle to prevalence thereof. It is necessary for cost reduction of silicon crystal solar cell to decrease production cost of the substrate, and improve conversion efficiency. Accordingly, for reducing the cost of substrate materials, a cone part, a tail part and the like have been used as raw materials, which could not be used for electronic purpose such as production of semiconductor devices or which could not be useful product of single crystal ingot. However, acquisition of such a raw material is unstable, and the amount is limited. Accordingly, in light of increase of a demand on silicon single crystal solar cell, it is difficult to produce a necessary amount of solar cell substrates stably by such a method.
It is also important in the solar cell industry to produce a solar cell having wider area in order to obtain more electric current. CZ method is suitable for producing a silicon wafer with a large diameter which can be substrate materials for production of a solar cell having wider area, since a silicon single crystal having a large diameter can be easily produced according to CZ method, and the produced silicon single crystal is excellent in strength. Accordingly, a silicon single crystal for a solar cell is mainly produced according to CZ method.
A silicon wafer can be used as material for substrate of single crystal solar cell, only where its substrate lifetime (hereinafter occasionally referred to as lifetime, LT), that is one of characteristics thereof, is more than 10 xcexcs. Furthermore, the lifetime is preferably 200 xcexcs or more in order to provide a solar cell having a high conversion efficiency.
However, concerning a single crystal produced by CZ method being at present a main method for producing a single crystal ingot, when it is radiated with a strong light after it is processed to be a solar cell, the lifetime of the solar cell substrate is lowered resulting in photo-degradation, so that it is required to be improved also as for performance of the solar cell.
It is known that boron and oxygen existing in the single crystal substrate cause lowering of life time and photo-degradation that is occurred when a solar cell produced using the silicon single crystal produced by CZ method is irradiated with strong light. A conductive type of the wafer that is presently used as a solar cell is mainly P type, and the P type wafer is generally doped with boron as a dopant. Although the single crystal ingot that is material for the wafer can be produced according to CZ method (including MCZ (hereinafter occasionally referred to as Magnetic field applied CZ method) or FZ method, production cost in FZ method is higher than in CZ method, and the silicon single crystal having a large diameter is produced more easily according to CZ method as described above. Accordingly, it is presently produced according to CZ method wherein a single crystal having a large diameter can be produced at relatively low cost.
However, oxygen exists at high concentration in the crystal produced according to a CZ method, and thus there is a problem that boron and oxygen in P type silicon single crystal produced according to CZ method may affect the lifetime characteristic of the solar cell substrate and may cause photo-degradation.
The present invention has been accomplished to solve the above-mentioned problems, and an object of the present invention is to provide a silicon single crystal and a silicon single crystal wafer for producing a solar cell having very high conversion efficiency of optical energy that is not suffered from photo-degradation, even though it has high oxygen concentration, and also provide a method for producing them.
To achieve the above object, the present invention provides a silicon single crystal doped with Gallium wherein resistivity is 5xcexa9.cm to 0.1xcexa9.cm.
The present invention also provides a silicon single crystal doped with Ga wherein concentration of Ga in the crystal is 5xc3x971017 atoms/cm3 to 3xc3x971015 atoms/cm3.
Although the substrate of the solar cell is desired to be a substrate having low resistivity and long lifetime, in the substrate wafer having extremely low resistivity, the lifetime of minority carrier becomes shorter due to Auger recombination, resulting in lowering of conversion efficiency. Accordingly, the amount of Gallium contained in the silicon single crystal of the present invention is preferably such an amount that the resistivity is 0.1xcexa9.cm or more, more preferably 0.2xcexa9.cm or more. Alternatively, concentration of Gallium is preferably 5xc3x971017 atoms/cm3 or less.
In the following description, the lifetime of such a carrier generated in the substrate is called substrate lifetime or lifetime.
On the other hand, too high resistivity of the substrate may also cause a problem. If the substrate resistivity gets high, electric power is consumed with internal resistance of the solar cell when it is processed to be a solar cell, so that conversion efficiency may also be lowered. For these reasons, if the wafer of the present invention is used as material for a substrate of a solar cell, it is preferable that the resistivity is 5xcexa9.cm or less, more preferably 2.0xcexa9.cm or less, or that gallium concentration in the single crystal ingot is 3xc3x971015 atoms/cm3 or more.
The present invention also provides a silicon single crystal doped with gallium wherein concentration of interstitial oxygen in the single crystal is 20xc3x971017 atoms/cm3 (ASTM""79) or less.
As described above, according to the present invention, if oxygen is contained in the crystal, photo-degradation can be prevented by Ga, so that oxygen concentration in the crystal can be as high as that in a general single crystal produced according to CZ method, especially can be high as 20xc3x971017 atoms/cm3 or less. Accordingly, it is not necessary to make oxygen concentration low forcedly, so that there are advantages that the single crystal can be produced. easily, and that strength of the crystal is high since adequate amount of oxygen is contained therein.
The present invention also provides a silicon single crystal doped with Ga wherein a diameter of the single crystal is 4 inches or more.
In the case that a diameter of a single crystal used for a substrate is large, the crystal produced according to CZ method or MCZ method tends to have high oxygen concentration. Accordingly, if a solar cell having high conversion efficiency is to be produced, it has been a general way to produce a single crystal according to FZ method or to produce a single crystal having a small diameter according to MCZ method, in order to achieve low oxygen concentration. However, it is very difficult to produce a single crystal having a diameter of more than 6 inches according to FZ method, and it is also difficult to produce a single crystal having a diameter of more than 4 inches and low oxygen concentration according to MCZ method. Accordingly, it has been considered as inappropriate to produce a single crystal having a large diameter for production of a solar cell having high conversion efficiency.
However, according to the silicon single crystal of the present invention, even if oxygen concentration in the single crystal is high, stable substrate lifetime can be achieved as described above. Accordingly, a single crystal ingot having a large diameter as 16 inches or 20 inches, which is not used at present can be used as a substrate wafer of a solar cell, so that a solar cell having high conversion efficiency can be easily produced irrespective of the diameter of the single crystal ingot. Furthermore, since a wafer having a large diameter out of use at present can be used as a substrate of a solar cell, a solar cell itself can be made large, so that it is fully possible to find more use of a solar cell.
The present invention also provides a silicon single crystal wafer doped with gallium produced by slicing a silicon single crystal doped with gallium produced according to Czochralski method.
If the silicon single crystal wafer doped with gallium is used as a material for a substrate of a solar cell, shortening of the lifetime due to influence of oxygen contained in the crystal can be suppressed. Accordingly, even if the single crystal wafer has high oxygen concentration, a long lifetime that is necessary for a solar cell can be achieved. Thereby, a long lifetime can be achieved even in a cell having a low resistivity, and it has become possible to produce a solar cell wherein a substrate wafer having high oxygen concentration is used, but conversion efficiency is not low, and high performance can be achieved. Furthermore, since oxygen concentration in the wafer is appropriate, there can be achieved an advantage in use that a strength of the wafer is high.
In order to use the silicon single crystal wafer doped with gallium as a solar cell substrate, the resistivity is preferably 5xcexa9.cm to 0.1xcexa9.cm, and is more preferably 2.0xcexa9.cm to 0.2xcexa9.cm. Alternatively, Ga concentration in the wafer substrate is preferably 5xc3x971017 atoms/cm3 to 3xc3x971015 atoms/cm3, more preferably 1.5xc3x971017 atoms/cm3 to 7xc3x971015 atoms/cm3.
If a wafer having resistivity more than 5xcexa9.cm, or having Ga concentration smaller than 3xc3x971015 atoms/cm3 is used as a substrate of a solar cell, resistivity of the wafer will be unnecessarily high, and an electric power is consumed with an internal resistance of the solar cell and conversion efficiency may be reduced, when the substrate is processed to be a solar cell. If a resistivity of the wafer is less than 0.1xcexa9.cm, or Ga concentration in the wafer is more than 5xc3x971017 atoms/cm3, resistivity of substrate will be extremely reduce and, as a result, lifetime of minority carrier will be lowered due to Auger recombination, and conversion efficiency will be lowered. Accordingly, it is preferable that a wafer having resistivity of 5xcexa9.cm to 0.1xcexa9.cm, or has concentration of Gallium in the range of 5xc3x971017 atoms/cm3 to 3xc3x971015 atoms/cm3.
Interstitial oxygen concentration in the silicon single crystal wafer doped with Ga of the present invention is preferably 20xc3x971017 atoms/cm3 (ASTM(79)) or less. Namely, it is acceptable in the case that oxygen concentration in the substrate of the solar cell doped with Ga is as that of a single crystal produced according to CZ method. Namely, a silicon wafer having oxygen concentration of a value incorporated in the crystal during the growth of single crystal, namely a value less than solid solubility of oxygen at a temperature around a melting point of silicon is suitable.
In order to obtain a silicon single crystal wafer having oxygen concentration more than 20xc3x971017 atoms/cm3, a silicon single crystal ingot having so high oxygen concentration as to meet the necessity. In order to obtain a single crystal ingot having high oxygen concentration more than necessary, it is necessary to select the condition of production under which a single crystal ingot is hardly produced, for example, high speed of rotation of crucible during the growth of single crystal. Under such condition of crystal growth, slip dislocation may be generated in single crystal during the growth of the single crystal, or the crystal cannot be pulled straight, resulting in deformation of the crystal, so that some crystal cannot be processed to be a substrate of a solar cell. As a result, production cost of a wafer gets high, and economical merit is hardly obtained. Accordingly, oxygen concentration of a wafer used for the present invention is preferably 20xc3x971017 atoms/cm3 or less.
As described above, the silicon single crystal doped with gallium and the silicon single crystal wafer doped with gallium of the present invention is especially useful for a solar cell.
The silicon single crystal solar cell produced using Ga doped silicon single crystal or Ga doped silicon single crystal wafer is inexpensive and has high energy conversion efficiency.
Namely, if Ga doped silicon single crystal ingot grown, for example, by CZ method is proc ssed to be a substrate for a solar cell, and a solar cell is produced using the wafer, a solar cell having stable conversion efficiency can be produced without being affected by concentration of oxygen incorporated in the crystal during the growth of the silicon crystal. If the Ga doped silicon single crystal is used as material for a solar cell, the substrate lifetime can be stable without being affected by concentration of oxygen, so that there can be produced a solar cell having high conversion efficiency even when resistivity of the solar cell is low.
In conventional boron doped single crystal, as the resistivity becomes lower, so the lifetime becomes shorter. Accordingly, a solar cell having high conversion efficiency and low resistivity could not be produced. However, using Ga doped silicon single crystal and silicon single crystal wafer of the present invention, a solar cell having high conversion efficiency can be produced.
In that case, the above-mentioned silicon single crystal solar cell can have an area of 100 cm3 or more.
As described above, if Ga doped silicon single crystal according to CZ method is used as a substrate for a solar cell, a solar cell having high conversion efficiency, less lowering of conversion efficiency due to photo-degradation and large area can be produced at low production cost, so that cost of solar cell can be further reduced and increase of demand thereon may be expected. Furthermore, if the area is large, more electric current can be obtained from one cell, so that it is also effective for electric power.
Furthermore, the above-mentioned silicon single crystal solar cell has preferably conversion efficiency of 20% or more.
As described above, if Ga doped silicon single crystal is used as a substrate of a solar cell, a solar cell having high conversion efficiency and less lowering of conversion efficiency due to photo-degradation can be produced. In that case, conversion efficiency can be increased 20% or more. Especially, it should be pointed out that conventionally a solar cell having an area of 100 cm2 or more and conversion efficiency of 20% or more could not be in practical use, however, according to the present invention, 20% or more of conversion efficiency can be achieved in the solar cell having the cell area of 100 cm2 or more.
The silicon single crystal solar cell of the present invention can be for space use.
Since the silicon single crystal solar cell of the present invention is made of Ga doped single crystal, it is not susceptible to various radiation in space. Namely, rapid photo-degradation as observed in boron dope crystal is not observed. Accordingly, the silicon single crystal solar cell is suitable for space use.
The silicon single crystal solar cell described above may have a rate of lowering of conversion efficiency due to photo-degradation of 0.5% or less.
As described above, since the silicon single crystal solar cell of the present invention has high conversion efficiency and almost no lowering of conversion efficiency caused by photo-degradation, it is quite effective for a solar cell.
In the above description, xe2x80x9clowering of conversion efficiency due to photo-degradationxe2x80x9d represents a value obtained by subtracting conversion efficiency after irradiation of steady-state irradiation with halogen lamp or the like used for solar simulator for 30 hours from conversion efficiency before the irradiation.
The present invention also provides a method for production of GA doped silicon single crystal according to CZ method wherein Ga is added in a silicon melt in a crucible, a seed crystal is brought into contact with the silicon melt and is pulled with rotating to grow a silicon single crystal ingot.
Thereby, Ga doped silicon single crystal can be produced.
In that case, addition of Ga to a melt in a crucible is preferably conducted by growing a silicon crystal ingot in which Ga of high concentration is added previously, and crashing the silicon single crystal doped with Ga in high concentration to prepare a doping agent, and adding Ga in a silicon melt using it.
As a method of doping Ga in the case that a single crystal in which Ga is added is produced according to the present invention, a method of adding gallium directly to silicon polycrystal before or after being melted can be used, but in the case that the single crystal doped with gallium is mass-produced for industrial purpose, it is preferable that the doping agent is previously prepared to use for doping as described above. If such a method is used, operation can be conducted efficiently. Because, gallium has a low melting point as 30xc2x0 C., and is difficult to be handled. Accordingly, Ga concentration can be controlled accurately and easily not by adding gallium directly in a crucible, but by preparing a doping agent and doping with it, and an accurate dopant concentration can be achieved. Furthermore, a doping agent is easy to be handled, compared to the case that gallium is directly added in the silicon melt, so that operation efficiency can also be improved.
The number of rotation of a crucible during growing a Ga doped single crystal ingot can be 30 rpm or less.
An amount of oxygen eluted from a wall of a crucible can be controlled by changing the number of rotation of the crucible during growing a single crystal, and thereby an amount of oxygen incorporated in the single crystal to be grown can be controlled. However, even when Ga doped silicon single crystal is grown, the upper limit of the number of rotation of the crucible is 30 rpm, in light of ruffl of the liquid surface of the silicon melt or the like generated as a result of the vibration due to rotation of the crucible. Preferably the single crystal is grown with controlling the number of rotation of the crucible, depending on the intended oxygen concentration of the crystal to be pulled, in the range of 30 rpm or less. The upper limit of the number of rotation of the crucible is constant irrespective of the diameter of the single crystal to be pulled or the size of the crucible. Accordingly, if a single crystal is grown with controlling the number of rotation of the crucible in the range of 30 rpm or less, the single crystal can be grown efficiently without causing slip dislocation during the growth of the single crystal.
It is preferable that a pressure in a furnace of a pulling apparatus during the growth of Ga doped silicon single crystal is in the range of 10 to 500 mbar.
Oxygen eluted from a wall of a quartz crucible is always evaporated in the form of SiO from a surface of a silicon melt. Accordingly, in order to keep oxygen concentration in the silicon melt at a necessary value, a pressure in a chamber needs to be controlled appropriately. If the pressure in the furnace is 10 mbar or lower, extremely much amount of SiO is evaporated from the silicon melt, and an amount of oxygen eluted from the quartz crucible is increased, resulting in acceleration of degradation of the wall of the quartz crucible, so that the quartz crucible cannot stand against a long period of operation. Therefore, such a low pressure is not preferable. If the pressure is 500 mbar or more, SiO evaporated from the silicon melt is liable to adhered in the chamber, which may prevent growth of single crystal. Accordingly, unnecessarily high pressure is not preferable. It is preferable to decide a pressure in the furnace selected in the range of 10 to 500 mbar depending on the intended quality of the single crystal ingot to be produced, when Ga doped silicon single crystal ingot is grown.
An amount of inert gas to be flown in a furnace of a pulling apparatus during growing Ga doped silicon single crystal is preferably in the range of 10 to 500 l/min.
If the amount of inert gas flown above the liquid surface of melt is more than 500 l/min, an amount of SiO removed from the surface of the silicon melt is increased, resulting in acceleration of degradation of the wall of the quartz crucible. Furthermore, if a large amount of inert gas is brought into contact with the surface of the silicon melt, ruffle of the melt is enlarged, resulting in inhibiting growth of the single crystal ingot, which may lead to a problem that a single crystal cannot be pulled. If an amount of inert gas is less than 10 l/min, effect of removing SiO evaporated from the surface of the melt is lowered, there may be generated problems that cause dislocation generation during growing single crystal, for example, oxides of silicon may be precipitated on the upper part of the crucible. For these reasons, an amount of inert gas during growing single crystal is preferably controlled in the range of 10 to 500 l/min depending on intended quality of crystal.
It is preferable that the inert gas flown in the furnace of the pulling apparatus during growing Ga doped silicon single crystal is argon.
If argon is used as the inert gas flown in the furnace of the pulling apparatus during glowing the single crystal, any degradation of quality as will be problem in the solar cell is not caused, when the single crystal ingot is processed to be the solar cell, so that the substrate wafer having a stable quality can be produced, since Ar is chemically stable and does not affect the quality of the single crystal that is grown.
If the silicon single crystal and silicon single crystal wafer produced by Czochralski method is doped with Ga according to the present invention, photo-degradation is not caused even in the case that the single crystal having high oxygen concentration is produced, so that silicon single crystal and silicon single crystal wafer for producing a solar cell having very high conversion efficiency of optical energy can be produced. Besides, the present invention facilitates manufacture of these products with a large diameter, sufficient strength of crystal and excellent durability.